1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device with a flat-type fluorescent lamp capable of emitting light from both sides thereof.
2. Discussion of the Related Art
Cathode ray tubes (CRTs) have been widely used for monitors of televisions, measuring machines and information terminals. However, the CRTs cannot satisfy the demand for miniaturization and lightweight display devices. Thus, display devices such as liquid crystal display (LCD) devices that use an electro-optical effect, plasma display panel (PDP) devices that use a gas discharge, and electroluminescence display (ELD) device that use an electric-field-luminous effect have been developed to replace the CRTs.
Among the display devices, LCD devices have been most actively studied and developed because of such advantages as compact size, lightweight and low power consumption. Thus, LCD devices have become suitable for ultra-thin flat display devices, for example, monitors for spacecrafts, aircrafts, notebook computers, laptop computers, desktop computers and large-sized display devices, and the demand for LCD devices continuously increases.
Most LCD devices display images by controlling the transmittance of light. Thus, these LCD devices require an additional light source such as a backlight unit. In general, such a backlight unit has a cylindrical fluorescent lamp and is classified into two types, the direct type and the edge type, according to the arrangement of the fluorescent lamp.
In the edge-type backlight unit, a lamp unit is provided at one side of a light-guiding plate. The lamp unit includes a lamp for emitting light, a lamp holder inserted into both ends of the lamp for protecting the lamp, and a reflective sheet for reflecting the light emitted from the lamp toward the light-guiding plate. One side of the reflective sheet is inserted into the light-guiding plate and surrounds the circumference of the lamp. The edge-type backlight unit is generally used in relatively small-sized LCD devices for the monitors of laptop computers and desktop computers, and has such advantages as good uniformity, long life span, and thin profile.
In the direct-type backlight unit, a plurality of lamps are formed on a lower surface of a light-diffusion sheet, whereby an entire surface of the LCD panel is directly illuminated with the light. Because the direct-type backlight unit has greater light efficiency compared with the edge-type backlight unit, it is generally used in large-sized LCD devices (i.e., greater than twenty inches) which require high luminance. That is, LCD devices including such a direct-type backlight unit are generally used for large-sized monitors or televisions. When such a large-sized monitor or television that includes the direct-type backlight unit is turned on for a long time, some of the lamps inside the backlight unit may break down.
In case of an LCD device with the edge-type backlight unit, when one of the lamps breaks down, the LCD device can display images without a major problem, although the luminance of the LCD device is slightly degraded. However, in case of an LCD device with the direct-type backlight unit, a plurality of lamps are provided underneath a screen of the LCD panel. Accordingly, when one of the lamps breaks down due to trouble with the lamp or the end of the lamp life, the portion of the screen corresponding to the malfunctioning lamp portion becomes darker than the surrounding portions of the screen. Thus, the direct-type LCD device is required to have a simple structure suitable for disassemble and assemble of the lamp unit.
Hereinafter, a backlight unit according to the related art will be described with reference to the accompanying drawings.
FIG. 1 is a perspective view illustrating a direct-type backlight unit according to the related art. As illustrated in FIG. 1, the direct-type backlight unit includes a plurality of fluorescent lamps 1, an outer case 3 and light-scattering means 5a, 5b and 5c. The plurality of fluorescent lamps 1 are arranged at fixed intervals along one direction, and the outer case 3 fixes and supports the plurality of fluorescent lamps 1. The light-scattering means 5a, 5b and 5c are provided between the fluorescent lamps 1 and an LCD panel (not shown).
The light-scattering means 5a, 5b and 5c provide a uniform luminance to the LCD panel and prevent the silhouette of the fluorescent lamps 1 from being shown on the display surface of the LCD panel. In order to improve the light-scattering effect, a plurality of diffusion sheets and one diffusion plate may be provided between the fluorescent lamps 1 and the LCD panel. Also, a reflective sheet 7 is provided on an inner surface of the outer case 3 for directing the light emitted from the fluorescent lamps 1 toward the LCD panel, thereby improving the light efficiency.
Cold Cathode Fluorescent Lamps (CCFL) are used as the fluorescent lamps 1. Specifically, electrodes are provided at both ends of a tube in each of the fluorescent lamps 1. Accordingly, when a voltage is applied to the electrodes of the fluorescent lamp 1, the fluorescent lamp 1 emits light. Also, each end of the fluorescent lamp 1 is inserted into a hole formed on a side of the outer case 3. Then, both electrodes of the fluorescent lamp 1 are connected with power supplying wires 9 and 9a for operating the fluorescent lamp 1. Also, the power supplying wires 9 and 9a are connected with a driving circuit through an additional connector. Thus, each fluorescent lamp 1 requires an individual connector.
In the aforementioned direct-type backlight unit according to the related art, when a voltage is applied to both electrodes of the fluorescent lamp 1 through the power supplying wires 9 and 9a, the fluorescent lamp 1 emits light. Accordingly, the LCD panel is directly irradiated with the light emitted from the fluorescent lamp 1 through the reflective sheet 7 and the light-scattering means 5a, 5b and 5c. 
FIG. 2 is a perspective view illustrating an edge-type backlight unit according to the related art. As illustrated in FIG. 2, the edge-type backlight unit includes an LCD panel 14, a lamp 10, a light-guiding plate 1, a light-diffusion sheet 12, a prism sheet 13, a fixation structure 15, and a lower reflective sheet 16.
The LCD panel 14 displays images by controlling the transmittance of the light emitted from the lamp 10 that is used as a light source. The light-guiding plate 1 guides the light emitted from the lamp 10 toward the LCD panel 14. The light-diffusion sheet 12 diffuses the light guided by the light-guiding plate 1 to a predetermined angle. The prism sheet 13 concentrates the diffused light and directs the concentrated light toward the rear of the LCD panel 14. The fixation structure 15 is positioned below the light-guiding plate 1. To minimize light loss, the lower reflective sheet 16 reflects the light traveling toward the fixation structure 15 to the LCD panel 14.
The edge-type backlight unit according to the related art further includes lamp reflective sheets 18 and lamp holders 17. Each of the lamp reflective sheets 18 covers the lamp 10 except for a surface of the lamp facing the light-guiding plate 1, thereby decreasing light loss. The lamp holders 17 are positioned at both ends of the lamp 10 to support the lamp 10 and to maintain a predetermined interval between the light-guiding plate 1 and the lamp 10. A plurality of light-diffusion sheets and prism sheets may be provided between the LCD panel 14 and the light-guiding plate, instead of a single pair of light-diffusion sheet and prism sheet. The light emitted from the lamps 10 of the edge-type backlight unit is transmitted to the LCD panel through the light-guiding plate, the light-diffusion sheet and the prism sheet.
In the edge-type backlight unit according to the related art, the lamps 10 are positioned at both sides of the light-guiding plate 1. Such an edge-type backlight unit is generally used for large-sized monitors. However, when the edge-type backlight unit is used for a small-sized notebook computer, a single lamp may be positioned at any one side of the light-guiding plate 1.
The related art direct-type and edge-type backlight units can be used for notebook computers or monitors. However, when such related art backlight units are used for display devices that can display images on both sides, two LCD modules, each of which has a backlight unit, are required to construct such dual-emission display devices, thereby complicating the structure and increasing the thickness of the dual-emission display devices.